1. Field of the Disclosure
The disclosure relates to the field of acoustic measurement devices in oil exploration. Specifically, the disclosure is a method of measuring ultrasound velocity in drilling mud in a borehole formation.
2. Background of the Art
Borehole caliper is an important factor in the available accuracy and effectiveness of downhole data gathering instruments. Spatial irregularities in the borehole walls can adversely affect data integrity, unless these irregularities are detected and accounted for in data processing and/or acquisition. Borehole rugosity adversely affects downhole data measurements which are designed to assess the potential for hydrocarbon bearing formations adjacent a borehole. For example, cavities in the borehole wall can adversely affect measurements taken by downhole devices such as Nuclear Magnetic Resonance (NMR) devices. Thus, there is a need for an accurate downhole measurement of borehole rugosity. Resistivity measurements and gamma ray density measurements are also sensitive to the borehole size shape and standoff.
Ultrasonic pulse-echo measurements have long been used in wireline and logging-while-drilling (LWD) tools to measure a variety of parameters including instantaneous standoff, borehole caliper, or features on the borehole wall such as rugosity, fractures, or cracks.
The working principle for these downhole applications involves mounting one or more highly mechanically damped ultrasonic transducers on an (LWD) tool for use during a drilling operation. The transducer emits a short duration broadband pulse. The pulse then reflects from the surface being probed and returns and re-excites the emitting transducer. The transducer is positioned such that at least some of the acoustic pulse propagates through the surrounding man-made borehole fluid, commonly referred to as drilling mud.
Inaccuracy in the exact value of ultrasound velocity in the borehole fluids limits the accuracy of the measurement. The transit time τ for the echo determines the distance D to the reflecting surface. D=Vmud*τ. However, the accuracy of the conversion from transit time to distance traveled is limited by the accuracy of the value of ultrasound velocity in the drilling mud, Vmud. The ultrasound velocity in standard drilling mud is usually within 20% of that of water (1493 m/sec). Thus the propagation distance may have 20% inaccuracy. Higher accuracy is often required.
To date, measurement of ultrasonic sound velocity in drilling mud has been made using pulse transmission techniques in which acoustic pulses are transmitted through the drilling mud in the annulus between the drill collar and the borehole wall. See, for example, U.S. patent application Ser. No. 10/298,706 of Hassan et al., and U.S. Pat. No. 6,618,322 to Georgi et al., both having the same assignee as the present disclosure and the contents of which are incorporated herein by reference.
Pulse transmission techniques in the annulus are difficult to make. First, the drill cuttings being brought up by the drilling mud in the annulus include relatively large particles which scatter and attenuate the pulses. Consequently, the pulses received by the receiver are decreased in amplitude and include a lot of scattered noise. The drill cuttings are also highly abrasive leading to rapid wear and tear on the transducers. In configurations in which the transducers are positioned behind an acoustic window, the window itself can give rise to reverberations.
The disclosure herein discloses methods to measure ultrasound velocity and attenuation in drilling mud in an LWD environment. The device is particularly useful in applications where real-time mud velocity corrections are needed and cannot be applied after LWD tool use.